This invention relates to vehicles with multiple driven axles and particularly to agricultural and industrial tractors with front and rear driven axles.
Such tractors conventionally have a clutch for connecting and disconnecting drive to the front axle to switch between two and four wheel drive. This clutch is necessary since it is essential to disconnect drive to the front wheels in certain operating conditions, for example, when the tractor is being driven at speed on a road in order to prevent excessive wear to the tires due to unequal peripheral tire speed.
It is also necessary to dis-engage the clutch when the tractor is required to turn very sharply. In these circumstances, the front wheels have to travel appreciably faster than the rear wheels and the turn is impeded if the front wheels are mechanically driven at the same peripheral speed as the rear wheels.
It is well known on four wheel drive highway vehicles to use a differential between the front and rear axles, thus eliminating the excess tire wear at high speed and the restriction of sharp turns. However, this system is unsatisfactory on a tractor due to the normal variation in front/rear weight distribution, which can vary between 80/20 and 20/80.
It is also known to be advantageous in a tractor provided with rear axle brakes only, to ensure that drive to the front wheels is directly connected to the rear wheels whenever such a tractor is braked in order to ensure that both front and rear axles can contribute to the braking effect via their contact with the ground.
Various systems have therefore been proposed which, for example, take account of the requirement to disconnect front wheel drive at speed on the road and which ensure that when both rear axle brakes are applied drive is automatically connected to the front axle.
Other systems have been proposed which change the gear ratio between the front and rear axles when the tractor is turning sharply; this causes the peripheral speed of the front wheels to increase relative to the rear wheels so that the front wheel drive aids rather than impedes the sharpness of the turn.
It is an object of the present invention to provide a vehicle with multiple driven axles which takes account of the operating conditions of an agricultural or industrial tractor and which obviates the need for a clutch to connect or disconnect drive to the front axle and for an inter-axle ratio changing device to facilitate sharp turns, as discussed above.